Formation of protective layers on iron and steel articles



Patented Sept. 30, 1941 um-[ED STATES ronmTron or azsmss PROTECTIVE LAYERS N- IRON AND STEEL ARTICLES I v I Gottfried Becker, Dusseldorf, m mu .Steinberg, Krefeld, Germany No Drawing. Application March 12, 1937, Serial No. 130,586. In Germany November 10, 1934 1 Claim. (01. 91-401) 1 The present invention relates to improvements in or relating to thejformation of protective layers on iron or steel articles by forming a layer of chromium or a chromium alloy at or within the surface thereof, the forming of such a layer on the article surface for convenience in terminology herein called chromising the surface.

It is an objectof the present invention to provide an improved process for forming protective layers on iron and steel articles which can be carried out continuously in a controlled manner.

It is a further object of the invention to provide a method of forming protective layers on articles of iron or ferrous metals such as steel by treating them with gaseous chromous chloride at an elevated temperature.

It is a further object of the invention to provide a method of forming protective layers on articles of iron or ferrous metals such as steel by treating them in a gaseous mixture comprising chromous chloride and hydrogen.

It is a further object of the present invention to provide an improved method of forming protective layers on iron or steel articles by heating them to about 900 C. or over in an atmosphere resulting from the reaction of a mixture of hydrogen and hydrogen chloride on chromium or a chromium alloy especially ferrochrome.

An important feature of the present invention resides in the use of gaseous chromous chloride or the gas atmosphere above referred to as distinct from chromic chloride as such.

According to the present invention, we provide a method of forming protective layers on iron and steel articles including the steps of passing a mixture of hydrogen and hydrogen chloride over heated chromium or chromium alloys and causing or allowing the resultant gaseous prodnet to act on the articles at a temperature of about 900 C. or over.

The terms gas or gaseous" as used herein refer not only to true gases but also to vapours.

It is preferred to employ the hydrogen in a large proportion as compared with the quantity of hydrochloric acid. As a result of tests it is believed that the gaseous product contains chromium in the form of chromous chloride and indeed it has been found by extensive research that the formation of the chromium or chromium containing layer proceeds considerably more rapidly and more uniformly than is the case where chromic chloride is employed as such. Moreover in the method of the present invention the chromium containing gas or vapour can be proregulatable amounts or concentration) Moreover the chromium or chromium containing layer may be made of any desired thickness by con trolling the reaction conditions or time of treat- It is indeed surprising that the gaseous or vapourous-chromiumcompound which is believed to be chromous chloride is produced at all under the strongly reducing conditions because it is known that chromium compounds are reducedto metallic chromium when heated in the presence of hydrogen. When a chromium alloy, such for example as ferrochromeris employed it is probable that the hydrogenchloride first attacks the surfaceof the ferrochrome forming volatileferrous chloride which of course has no action. on the articles to be treated. As the iron is removed from the surface of the ferrochrome leaving chromium, the latter is then attacked by the hydrogen chloride in the presence of hydrogen to form a volatile chromium compound, probably chromous chloride, which as stated above, reacts particularly favourably with the iron or steel. The resulting gas mixture may be passed continuously over the articles to be treated at a regulatable rate whereby the desired exchange of chromium and iron is effected. The chromium penetrates or diffuses into the surface of the articles without substantially altering the dimensions of the original articles. At a temperature of 980 C. and a treating period of 4 to 6 hours a chromium containing layer of a thickness of about 0.1 mm. has been obtained. The articles treated had a smooth lustrous surface and the chromium content of the protective layer was about 25%.

The hydrogen and the hydrogen chloride can be produced or supplied from separate sources and mixed with one another or the mixture of hydrogen chloride and hydrogen may be conveniently produced by passing hydrogen at a reg ulated rate through fuming hydrochloric acid.

Either or both of the gases hydrogen chloride and hydrogen or the mixture of the two, may be passed through one or more suitable drying means for example bubbled through concentrated sulphuric acid or passed through drying tubes or towers. This is especially advantageous when producing the mixture of the two gases by bubbliing hydrogen through fuming hydrochloric ac d.

One of the advantages of the method according to the present invention is that by varying the composition of the gas mixture, 1. e. the conduced continuously and brought to reaction in centration of chromium compound, for example process can be interrupted at any desired instant.

If desired the articles to be treated, may first be subjected to the action of hydrogen alone whilst being heated for the purpose of removing surface impurities such as oxides. It is therefore generally not necessary to clean the articles before introducing them into the reaction chamber as has previously been considered necessary.

The method of the present invention may be carried out in various ways. The volatile chromium compound can be produced in the same vessel as the article or articles to be tneated or in different vessels. Thus for example chromium or an alloy thereof for instance ferrochrome may be placed in one or more retorts or drums and heated to red heat whilst a mixture of hydrogen chloride and hydrogen is passed over it. The gaseous product which is probaly a mixture of chromous chloride and hydrogen may then be passed through a drum or other suitable vessel containing the article or articles to be treated, a

drogen is caused to act on thechromium or chromium alloy preferably ierrochrome in the same chamber or vessel as that in which the article or articles to be treated are placed, but the arrangement should be such that the stream of hydrogen and hydrogen chloride meets the chromium or chromium alloy before reaching the articles to be treated. In this case the chromium or chromium alloy and/or the article or articles to be treated, may be enclosed in suitable perforated or cage like containers. If desired the article or articles to be treated may be placed in a rotary perforated or cage like container arranged within a larger container in which the chromium or ferrochrome is placed. Alternatively a rotary container may contain both the article or articles to be treated and the chromium or chromium alloy, these being separated if desired by a perforated or grid or sievelike partition.

We claim:

A method of chromising the surface portions of iron and steel articles comprising passing a gaseous mixture consisting essentially of hydrogen and chromous chloride from a source of supply through a heated chamber containing the articles so as to cause a stream of said mixture to pass continuously over the articles in contact therewith, whereby said surfaces will be continuously acted upon by a mixture of hydrogen and chromous chloride for a chromising treatment of any duration, the chamber being heated to such temperature as will maintain the articles GOTTFRIED BECKER. FRITZ STEINBERG. 

